displays four CO2 adsorption
cycles for CASIL-SBA-B3-[TBA][Tau]-55 at 25 °C. After
each cycle, the material was degassed for 16 h under vacuum at
110 °C. As can be seen, the CO2 capacity is slightly decreased
with each recurring cycle. This can likely be attributed to the
inability to remove all captured CO2 at these regeneration conditions,
and each subsequent regeneration step likely continues
to remove small amounts of water from the composites, which
decreases the capacity of the materials consistent with the data
shown in Figure 8. As seen in the previous section, water
concentration appears to significantly impact CO2 capacity in
these composites. While the majority of the water is likely
removed during the degas process, the removal of trace
amounts may need higher temperatures and/or longer degas
hold times.
In addition to overall CO2 capacity, water stability, long-term
material stability, and regeneration energies are of significant
importance in potential application of novel CO2 sorbent
materials. As displayed in the previous section, while the
[TMA][Tau] TSILs are advantageous due to their lower molecular
weight and thus a higher number of amine groups impregnated per
composite, the CO2 equilibrium isotherms displayed much lower
capacities per amine present than the impregnated longer alkyl
chain TSILs composites. However, as displayed in Figure 10,
elevated temperatures are shown to increase CO2 capacities of
three CASIL-[TMA][Tau] materials. Interestingly, temperature
effects of the other four TSIL composites displayed in Figure 11
follow a more expected trend with decreasing CO2 adsorption with
increasing isothermal temperature.
Although, the trend of increasing temperature resulting in an
increase in adsorption capacity is observed on all three CASIL-
[TMA][Tau] samples, a general trend reflecting an optimum
temperature for adsorption is absent. The lack of a clear trend
of capacity as a function of temperature is somewhat expected
as the CO2 capture mechanism for these composites is complex.
For example, it has been shown that increasing the temperature
only 20 deg in neat tetra-n-alkylammonium amino acid
salts decreases the viscosity by up to a third of the initial 25 °C
viscosity.46,77 Additionally, the hydrogen bonding formed
during the ammonium carbamate formation during CO2 complexation
by the amino acid TSIL has been shown to significantly
increase the viscosity. Ren et al. found the neat dual
amine containing TSIL swelled and solidified, resulting in an
increase molar volume upon the complexation of CO2.
61 It is
also possible that the isotherm temperature, coupled with any
trace water in the ionic liquid, may melt the ionic liquid
contained in the pores. The impact of temperature is observed
on each of the three CASIL-[TMA][Tau] materials all of which
were made with different sized porous supports indicating that
the effect is not limited to one sample.
Therefore, one possible explanation for the low CO2
capacities and amine efficiencies of the [TMA][Tau] composites